Automobile and truck bodies are treated with multilayer coating systems that enhance the aesthetic appearance of the vehicle and also provide protection from prolonged exposure to the environment or weathering. Basecoat/clearcoat finishes for automobiles and trucks have been commonly used for the past two decades. U.S. Pat. No. 4,728,543 and U.S. Pat. No. 3,639,347 disclose the application of a transparent protective clearcoat over a color coat or pigmented basecoat in a “wet on wet” application, i.e., the clearcoat is applied before the basecoat is completely cured. At the time of this application, it has become popular to produce finishes on vehicles that are either solid color, pearlescent color, or have a metallic sparkle. The metallic sparkle finishes are also described in the art as metallic effect finishes, which utilize a metallic flake pigment, such as aluminum flakes, in the pigmented basecoat layer to impart a glamorous, high gloss, metallic appearance.
As is well known in the art, in producing a finish of automotive quality on a substrate, multiple layers of coatings are generally used. A typical automobile steel panel or substrate has, for example, several layers of coatings. The substrate is typically first coated with an inorganic rust proofing zinc or iron phosphate layer over which is provided a corrosion resistant primer, which can be an electrocoated primer or a repair primer. A typical electrocoated primer, which is mainly used in original equipment manufacturing (OEM) applications, comprises epoxy polyester and various epoxy resins. Typically, a primer surfacer and/or sealer can be applied over the electrodeposited primer to provide a smooth surface for better appearance and to which the overlying layer or layers of basecoat will readily adhere. The cured primer layer can be sanded to remove any defects present such as, for example, dust particles in or on the primer or other imperfections.
In conventional practice, pigmented basecoat layers and/or “effect” basecoat layers are applied over the primer layer. Multiple basecoat layers can be applied depending upon the color and effect(s) that are desired in the finished product. The multiple layers can either be the same as each other or different from each other. In applications where two different basecoat layers are applied, such as for example when a metal flake pigment layer is applied over a solid color pigment layer, it is a requirement in the conventional art to use a forced drying or a curing step to rapidly dry or cure the first layer prior to application of the second layer. If the first basecoat layer is cured, the coated substrate is subjected to conditions such as, for example, curing ovens, that cause crosslinking of the film forming binder. When rapid drying is required in a painting process, it is conventional to use equipment such as blowers and/or heaters to remove at least 50% of a dispersant solvent from a first coating layer prior to application of a second coating layer having a different composition during the period of time allotted for painting a vehicle substrate. Simple evaporation under ambient conditions or reliance upon movement of air around a substrate as the substrate is moved along a paint process line is not adequate for rapid drying processes and, therefore, are not used conventionally where rapid drying processes—such as forced drying—are required.
Metal flake pigments come in a variety of shapes and sizes. Conventional effect finishes typically are relatively thick metal flake pigments that impart a sparkle to the finished substrate. Several pigment manufacturers produce thin metal flake pigments. It is possible to produce coatings using these thin metal flake pigments that have a polished or anodized metal look without the metallic sparkle imparted by thick metal flake pigments. However, thin metal flake pigments are difficult to use in a coating composition as they allow underlying surface defects, such as sanding scratches, to be readily visible in the finished substrate.
It is believed that the thin metal flake pigments are thin and flexible enough so as to conform to the topography of the underlying substrate. In this manner, during the solvent evaporation process, if thin metal flake pigments encounter a surface defect such as a sanding scratch, the flakes align and follow the topography of the defect. When the coating is dried and cured, the defect can be readily visible to the naked eye. In contrast, traditional metal flake pigments are thick enough so that they are able to lie flat on a substrate surface and bridge any sanding scratch defect without deforming. Such small scratches do not significantly affect a basecoating composition containing traditional metal flake pigments. Such traditional metal flake pigments typically have an average thickness of about 300 to about 500 nanometers whereas the thin metal flake pigments have an average thickness of only about 10 to 100 nanometers.
The desire for even more unique and attractive color effects has led the auto industry to find ways to create even brighter metallic effects than available today. Vapor metallized flake (VMF) pigments have been used in the basecoat layer to impart to the finish an extremely smooth and fine-textured bright metallic appearance similar to polished metal or an anodized metal with no perceptible “sparkle”. This polished or anodized metal appearance differs from traditional metallic finishes in that there is a minimum of perceptible sparkle and the human eye cannot readily discern individual metallic flakes.
In conventional practice, a polished or anodized metal appearance (also conventionally known, and referred to herein, as the “polished metal effect”) can be obtained using organic solvent-borne paints. However, use of such paints requires a large amount of surface preparation, and the paints are very difficult to produce. In a 2003 pre-print publication of Kansai Paint entitled “Super Metallic Silver Colors” (Y. Mizutami, Y. Nakao, and S. Nakamura) such a process is described where eight layers of paint and six bakes are required. This process would require substantial reconfiguration of a conventional paint line. European Patent Application 1,591,492 A1 discloses a coating method for forming a chrome-like coating effect, but requires that the effect layer be applied in multiple very thin coating layers. Such an application method is not useful in an OEM automotive facility without substantially slowing down the manufacturing process. U.S. Pat. No. 6,331,326 discloses a method for producing a plated metal effect on a substrate.
It would be desirable to eliminate multiple spray passes and baking steps in a process for manufacturing a vehicle having a polished metal appearance.